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Phillip Hughes played in 26 Tests for Australia before his death at the age of 25

Phillip Hughes collapsed after being hit by a cricket ball at the top of his neck and the base of the skull.

One of the major arteries into the brain split and caused massive bleeding.

The brain is a very delicate and vulnerable organ, which is surrounded by the skull - a defensive wall of bone - and a cushioning fluid.

But it is not a simple case of the brain having specific weak spots.

"The brain is protected by the skull, but the upper neck is vulnerable," said Peter Hutchinson, a professor of neurosurgery at University of Cambridge.

He told the BBC: "We evolved to enable a neck with a lot of movement, but the consequence is a risk of injury to the vessels from excessive movement or direct trauma."

The vertebral arteries run up both sides of the neck and the blow from the ball caused a dissection, in which the artery ruptures.

Antonio Belli, professor of trauma neurosurgery at the University of Birmingham, said: "The vertebral arteries supply the brain stem, which controls breathing and heart rhythm, so I think that could be why he stopped breathing immediately."

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How the brain is fed by arteries (in red) and veins (in blue) in the head and neck

Types of injury

While vertebral artery dissections are rare, there are two common types of brain injury known as "focal" and "diffuse".

Focal injuries tend to be caused by falls and assaults. They are the result of a damage to one area of the brain, which results in a blood clot.

This occupies space and causes pressure on the surrounding brain, starving the organ of oxygen leading to brain cells dying. It is treated by removing the clot.

However, damage may not be confined to just the site of the impact - known as a "coup injury".

The brain floats inside the skull so can collide with the side of the skull opposite the impact causing a "contra-coup injury".

The front of Olympic rower James Cracknell's brain was damaged when he was struck on the back of the head while cycling.

Widespread

The other class of injury is largely the result of high speed road accidents.

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The latest model of cricket helmet, which is said to be more protective, worn by New Zealand cricketer Ish Sodhi

In diffuse injuries the damage is spread across the brain, which becomes swollen.

The pressure builds in the tight confines of the skull and the flow of blood can be impaired.

Treatment requires lowering the pressure in the brain.

It is also possible to damage the brain without a direct blow to the head.

The brain sits in fluid inside the skull and can bounce around if there is enough force.

In something like severe whiplash the brain can be shaken around the skull, even though no blow is delivered.

Mr Belli told the BBC: "Often what does the damage is not the direct impact, but rotational forces.

"You could argue from an evolutionary point of view, we're well designed to withstand a direct blow, but not engineered well to withstand the rotational forces in a road traffic accident."

Thinner walls

The skull itself does have stronger and weaker parts.

A region called the pterion, close to the ears, is the thinnest part of the skull so is most vulnerable to fracture.

But the relatively reinforced forehead and back of the skull can still be damaged.

"They may be thicker parts of the skull, but you can still fracture them if you transmit enough force," Prof Hutchinson noted.

"There isn't a safe point or a weak point, you get a lot of damage from the way the force travels through the head," added Mr Belli.

Lasting damage

Damage varies hugely from one patient to another - one patient who has a seemingly severe injury can show better recovery than injuries which appear minor.

Prof Hutchinson said it the lasting damage depended on where the damage was caused as "some areas are more critical".

"Bleeding in the motor cortex can result in paralysis while damage to the visual cortex would result in blindness."

He said sport was not taking head injury seriously enough, but changes in American football were driving reform.

"In terms of sport, the most controversial is boxing because it is a deliberate act, horse riding creates a lot of serious injuries, and rugby increasingly so.

"There are racing drivers who have had an accident in the early part of the weekend, raced the whole weekend and not remembered a thing."